In a single speed self-propelled lawnmower, the engine is generally indirectly coupled to a set of drive wheels to propel the lawnmower at a single speed, which is usually related to the engine speed. Typically, a drive pulley is mounted to an output shaft of the engine and a belt or other transmission mechanism connects the drive pulley to a drive shaft which in turn rotates the drive wheels. When the drive system is engaged, the lawnmower automatically assumes the single speed. When the drive system is disengaged, the lawnmower stops.
If any variable speed control exists, it is typically accomplished by changing the engine speed. Unfortunately, this can be disadvantageous in situations where the operator might prefer a higher engine speed and a lower lawnmower speed, such as when cutting tall grass. If the engine speed is increased to provide for increased speed of the cutting elements and better cutting, the lawnmower moves faster and thereby negates the benefit of the increased speed of the cutting elements. Moreover, for some people, the single speed may be too fast or too slow and, therefore, an uncomfortable speed for walking behind a lawnmower. A variable speed mechanism that would allow the operator to adjust the speed of the lawnmower to suit the individual needs of the operator and separately control engine speed would be an improvement over conventional single speed drive systems.
In variable speed, self-propelled lawnmowers, two separate mechanisms are generally used to operate the lawnmower. An engagement mechanism is actuated to engage the drive system to begin propelling the lawnmower and a separate speed control mechanism is utilized to adjust the speed of the lawnmower. Moreover, conventional variable speed drive systems utilize complex and costly multiple gear sets or friction drive wheels and a clutch. A variable speed drive system that eliminates the complex and costly mechanisms yet uses a single mechanism to set the desired speed and to engage the drive system would provide a substantial improvement over conventional drive systems in the form of reduced manufacturing costs and maintenance requirements.
Variable speed drive systems have been developed in an attempt to provide variable speed to the drive of a self propelled machine without changing the speed of the engine. For example, a variable speed drive system has been developed for use with a lawnmower having an engine mounted to a frame and a pair of drive wheels coupled to an axle shaft for propelling the lawnmower. The system includes a variable pitch pulley attached to a pulley shaft that is in parallel with the axle shaft of the pair of drive wheels. The variable pitch pulley is coupled by a belt to a drive pulley mounted on an output shaft of the engine that is separate and distinct from the drive shaft operated by the engine to rotate the blade of the lawnmower. The output shaft on which the drive pulley is attached is also in parallel with the axle shaft. The variable pitch pulley is coupled to the axle shaft by a chain housing fixture that includes a chain and a sprocket. The fixture positions the variable pulley in spaced-apart relation to the axle shaft, and moves about the drive shaft between an engaged position and a disengaged position.
Such a variable drive system requires either two separate engines, one to drive the blade of the lawnmower and one to drive the output shaft for drive the wheels, or a complicated drive system that converts the rotation of the drive shaft of the blade to a rotation that is perpendicular thereto to rotate the output shaft. Further, this drive system occupies precious space on the lawnmower deck.
Therefore, in light of the above, a need exists for transmissions, drive systems and related methods that allow for easier control of propulsion of a self-propelled machine while reducing the complexity of the drive system and conserving space on the machine.